typescript challenging syntax within mix-ins

Question

typescript challenging syntax within mix-ins

type Constructor<T> = new (...args: any[]) => T;

function f1<T extends {}>(naked: Constructor<T>): any {
    return class dressed extends naked { } // error
}

function f2<T extends Constructor<{}>>(naked: T): any {
    return class dressed extends naked { } // ok
}

f1 mentions that

'dressed' can be assigned to the constraint of type 'T', but 'T' may potentially be instantiated with a different subtype of constraint '{}'.</p>
<p>I believe there isn't any logical issue in <code>f1

, so why is there a syntactical problem?

typescript class mixins

Answer 1

Answer №1

One aspect that can be perplexing is the Constructor<T> type because we are unable to define its generic argument T in the class mixin syntax.

To simplify and avoid using generics, we have the option to create a class mixin like f2:

type Constructor = new (...args: any[]) => any;

function Dressed<T extends Constructor>(Base: T) { // Note: no `: any` return type so that the type inference works.
  return class extends Base {}
}

In cases where a generic Constructor type is required elsewhere, it is possible to set a default type to continue using <T extends Constructor> without confusion:

type Constructor<T = {}> = new (...args: any[]) => T;

Answer 2

One aspect that can be perplexing is the Constructor<T> type because we are unable to define its generic argument T in the class mixin syntax.

To simplify and avoid using generics, we have the option to create a class mixin like f2:

type Constructor = new (...args: any[]) => any;

function Dressed<T extends Constructor>(Base: T) { // Note: no `: any` return type so that the type inference works.
  return class extends Base {}
}

In cases where a generic Constructor type is required elsewhere, it is possible to set a default type to continue using <T extends Constructor> without confusion:

type Constructor<T = {}> = new (...args: any[]) => T;

typescript challenging syntax within mix-ins

Answer №1

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